A compound semiconductor comprising a II-VI mixed crystal, such as a CdHgTe crystal, has the property that energy band gap thereof is changed by changing the composition ratio of the crystal, so that it is used in a low-noise photodetector and the like.
FIG. 4 is a schematic diagram for explaining a process for controlling conductivity type in a method for producing a II-VI compound semiconductor device, disclosed in Japanese Published Patent Application 62-34157. In FIG. 4, a CdHgTe crystal 2 is enclosed in a quartz ampoule 1 with mercury 3 which is liquid at room temperature. The mercury 3 is separated from the CdHgTe crystal 2 by the narrow part 1a of the ampoule 1, i.e., a mercury reservoir is provided at an end of the ampoule 1. Heaters 12 are disposed in the neighborhood of the ampoule 1.
Since mercury (Hg) has a very high vapor pressure, Hg is dissociated from the CdHgTe crystal 2 during the crystal growth, resulting in vacancies in the CdHgTe crystal. The vacancies serve as acceptors. For example, a CdHgTe crystal grown by liquid phase epitaxy (LPE) includes a lot of Hg vacancies and has a p type carrier concentration as high as 10.sup.17 cm.sup.-3, so that the mobility of carriers is low. Therefore, it is impossible to use the CdHgTe crystal for a semiconductor device. In order to solve this problem, as shown in FIG. 4, the CdHgTe crystal 2 is loaded in the ampoule 1 with mercury 3, the ampoule is evacuated to 10.sup.-6 Torr, and annealing is carried out at an appropriate temperature using the heaters 12. During the annealing, Hg partial pressure from the mercury 3, which is produced in the ampoule, is applied to the CdHgTe crystal 2 to fill the vacancies in the crystal, whereby the CdHgTe crystal having such a high carrier concentration is converted to a p type CdHgTe crystal having a low carrier concentration. If the Hg partial pressure is applied to the crystal until most of the vacancies are filled, an n type CdHgTe crystal is obtained.
The annealing process for reducing carrier concentration of the CdHgTe crystal will be described in more detail using the temperature profile of FIG. 5. The CdHgTe crystal 2 in the ampoule 1 is heated up to 280.degree. C. while the mercury 3 is heated to 250.degree. C., whereby an Hg partial pressure is applied to the CdHgTe crystal 2. The CdHgTe crystal 2 and the mercury 3 are kept in this state for about twenty four hours to diffuse Hg into the CdHgTe crystal 2 without Hg being applied to the inner wall of the ampoule 1, resulting in an n type CdHgTe crystal having a carrier concentration of approximately 10.sup.14 cm.sup.-3.
FIGS. 6(a) and 6(b) are schematic diagrams for explaining a method for producing p-n junctions in a II-VI compound semiconductor by diffusing mercury. A diffusion mask 4 comprising Zn and having a thickness of about 1 micron is disposed on a p type CdHgTe crystal 5 about 10 microns thick. A plurality of apertures 4a each having a diameter of about 1 to 30 microns are formed through the diffusion mask 4.
The CdHgTe crystal 5 with the diffusion mask 4 is loaded in a quartz ampoule with mercury as shown in FIG. 4 and annealed. During the annealing, the prescribed Hg partial pressure is applied to the surface of the CdHgTe crystal 5 exposed in the apertures 4a of the diffusion mask 4, whereby n type regions are formed as shown in FIG. 6(b), resulting in p-n junctions 6 at a depth of about 3 microns.
FIGS. 7(a)-7(e) are cross-sectional views of steps in a method for producing, for example, a photodiode array using the p-n junctions 6. After the p-n junctions 6 are formed in the CdHgTe crystal 5 as shown in FIG. 7(a), the diffusion mask 4 is removed as shown in FIG. 7(b). Then, an oxide film 101 is deposited on the entire surface of the CdHgTe crystal 5 as shown in FIG. 7(c). Subsequently, n side contact holes 102 are formed in film 101 at the n type regions and a p side contact hole 103 is formed in film 101 at on the CdHgTe crystal 5 using a conventional photolithography as shown in FIG. 7(d). Then, a metal layer is deposited on the wafer and patterned as shown in FIG. 7(e), resulting in n side electrodes 104 connecting to the CdHgTe crystal 5 through the contact holes 102 and 103 and a p side electrode 105 connecting to the CdHgTe crystal 5 through the contact hole 103.
In Japanese Patent Published Application No. 58-171848, a small quantity of group III element, such as indium, is added as a donor impurity during the growth of a CdHgTe crystal and then annealing is carried out to control the quantity of Hg in the CdHgTe crystal, whereby carrier concentration is controlled. Meanwhile, in Japanese Patent Published Application No. 62-13085, a bump electrode comprising indium is formed on a CdHgTe layer and then annealing is carried out to convert a region of the CdHgTe layer, on which the bump electrode is present, to n type. In these conventional methods, however, the indium as a donor impurity becomes a scattering center in the crystal, so that the mobility of carriers is not much improved, resulting in a semiconductor device with poor electrical characteristics.
In the above-described methods for producing semiconductor devices, the mercury, which is liquid at a room temperature, is used as a means for applying Hg vapor pressure to the II-VI crystal including mercury atoms. Therefore, if the ampoule is inclined, the mercury moves and it is difficult to obtain a desired temperature profile. Therefore, the ampoule should be treated with the greatest possible care, with the result that the work efficiency is unfavorably reduced.
In addition, as shown in the temperature profile of FIG. 5, the annealing should be carried out with the CdHgTe crystal 2 kept 30.degree. C. warmer than the Hg reservoir, to avoid Hg from adhering to the inner wall of the ampoule, and this results in a complicated process. In addition, when the group III element is used as a donor impurity, it acts as a scattering center and the mobility of carriers is not improved.